PROJECT SUMMARY/ABSTRACT Hearing loss is a leading cause of morbidity in children affecting about 2 to 3 out of every 1,000 individuals in the USA. While the study of affected genes has shed important insights into the mechanisms, we are just at the beginning to understand the disease mechanisms underlying morphogenetic events. The overall goal of this research is to elucidate the mechanisms that regulate the formation and maintenance of the stereociliary bundle of cochlear hair cells, and the defects in this process that cause deafness. We propose to investigate the functions of Fam65b protein complex in the cochlear hair cells and auditory perceptions. In our preliminary studies, we have identified several components in the Fam65b protein complex. We have generated genomic modified mouse models using CRISPR/Cas9 system. We have employed immunostaining and illustrated the stereociliary defects in these mice. Based on our preliminary data, we hypothesize that Fam65b protein complex is critical for the structural organization of stereociliary bundle of cochlear hair cells. To test our hypothesis, we will: i) Functionally characterize proteins in the Fam65b protein complex localized at the basal region of stereocilia; ii) Characterize the biochemical pathways by which Fam65b protein complex regulate hair cell function and determine the extent to which they modulate the actin cytoskeleton in stereocilia; iii) Continue our identification of hair cell proteins in the Fam65b protein complex. Our preliminary data show the feasibility of our approach. We anticipate that our studies will shed new insights into the molecular machinery that shapes stereocilia and determines its properties. Quite possibly, our findings may link several deafness-related genes into a common molecular pathway and provide new leads for the development of therapeutic approaches for the treatment of some forms of the disease.